In general, an optical fiber is composed of a core region and a cladding region. The core region is made of a silica glass of which a refractive index is increased by doping, for example, a germanium. The cladding region is made of a silica glass having a refractive index lower than that of the core region, which forms a layer surrounding a circumference of the core. Light propagates through the core region by a total reflection of the light at a boundary between the core region and the cladding region. Conventionally, a relative refractive index difference between the core region and the cladding region is at largest 3% to 4%.
On the other hand, in recent years, an optical fiber has been reported with which a large relative refractive-index difference can be obtained compared to the above structured optical fiber. For example, in Patent Document 1, it has been reported that an average refractive index of the cladding region can be greatly reduced by providing a microstructure such as a hole as a representative example in the glass of the cladding region, in a longitudinal direction. In other words, the optical fiber having such structure can dramatically increase an effective refractive index of the core region compared to the conventional optical fiber.
Given this situation, in recent years, an optical fiber having the microstructure has been a focus of constant attention, in which a hole or the like is formed in a surrounding area of the core region of an optical fiber having a refractive index profile structure equivalent to a typical single mode optical fiber (hereinafter, referred to as SMF). This optical fiber has an advantage in that the microstructure provided around the core region can increase the relative refractive-index difference between the core region and the cladding region, which cannot be achieved with an SMF, thereby making the macro-bending loss of the optical fiber extremely low.
For instance, it has been reported that the macro-bending loss against a small-diameter bending with a diameter of 15 mm could be lowered to 0.04 dB/m by adopting the microstructure (see, for example, Nonpatent Document 1). After that, there has been another report that the macro-bending loss could be further lowered up to below 0.01 dB/m (see, for example, Nonpatent Document 2). On the other hand, an optical fiber has been proposed, in which the macro-bending loss is lowered while maintaining a large mode field diameter (hereinafter, referred to as MFD) in view of connection with the conventional SMF (see, for example, Patent Document 2).
Patent Document 1: Japanese Patent No. 3306847
Nonpatent Document 1: T. Hasegawa, et al., Microoptics Conference (2003), K2
Nonpatent Document 2: Daizo Nishioka, et al., Shingaku Giho, OFT 2003-63, P. 23˜
Patent Document 2: Japanese Patent Application Laid-Open No. 2004 220026